This invention relates to method and apparatus for preparing and hydroforming parts and more particularly to prefilling the parts and then hydroforming the prefilled parts.
In the hydroforming of tubular metal parts, it is common practice as shown in FIGS. 1 and 2 of the accompanying drawings to employ a seal unit 10 located opposite each end of a tubular part 12 while the part is enclosed in a die cavity 14 formed by dies 16 and 18, only one such unit and one end of the part and dies being shown. The seal units include a piston 20 having a piston rod 21 to which a docking rod 22 is fixed. And the seal units are operated simultaneously to extend their docking rod to sealingly engage the respective end of the part with an interference fit when hydraulic fluid is supplied at pressure via a port 24 to a chamber 26 at one end of the piston while a chamber 28 at the other end of the piston is exhausted via a port 30.
This interference fit which is exaggerated in FIG. 2 for clarity is provided by a stepped cylindrical shoulder 32 near the end of the docking rod that has a relatively small diameter portion 32A that is closely received in the part and a larger diameter potion 32B of slightly larger diameter than the inner diameter of the part that is forced to enter the part to complete the sealed docking of the seal unit with the part. And the interference fit requires substantial axial force as well as adequate support about the outer diameter of the part where the docking rod enters. Moreover, the tubular part acts as a long slender column and must be prevented from buckling during this docking operation. Where buckling is a problem, conventional practice is to provide adequate confinement of the ends of the part in the hydroforming dies and to also preform the part to a certain compensating shape in special preforming dies prior to hydroforming the part in the hydroforming dies as further described below.
In the docking position, hydroforming fluid is supplied to fill the interior of the part through both seal units via a passage 34 in their docking rod. The pressure on this fluid is then gradually increased to expand the part outwardly while additional fluid is added with the part eventually being forced to conform to the die cavity surface wherein the part has been formed from the shape shown in phantom lines to the shape shown in solid lines in FIG. 1. As the part expands, the wall thickness tends to thin since a fixed amount of material in the part must now stretch to a larger dimension. To reduce or eliminate such wall thinning, the hydraulic pressure acting on the seal unit pistons is also gradually increased so that the piston force on the docking rods acting on the ends of the part exceeds the yield strength of the latter causing the part to shorten so that additional material from the part is supplied to the expanding portion of the part to minimize or eliminate such thinning.
Following the hydroforming of the part, the hydroforming fluid is drained from the part through the seal units and hydraulic fluid at pressure is then supplied to their chamber 28 while the other chamber 26 is exhausted to retract or withdraw their docking rod from the part. And the dies are then opened for removal of the hydroformed part.
While the above method and apparatus has proven generally satisfactory, a preforming operation as mentioned above is required in many cases to alter the cross-section of a round tubular part to for example a generally rectangular or square-shaped section in order to allow the part to freely enter the dies as they close about the part. If this preforming operation is not done in such cases, a part may be pinched at sections 12A and 12B between the mating surfaces 16A and 16B of the dies as shown in FIG. 3 as the dies close about the part. Furthermore, if such preforming is not done, it has been found that in many cases the part will have a tendency to buckle inwardly and will contact die cavity surface at multiple places when the dies are closed about the part as shown in FIG. 4. When this happens and the part is then filled with hydroforming fluid and this fluid is pressurized, the high pressure in the part causes high contact forces and thus high friction between the part and the die cavity surfaces at these places making it difficult, if not impossible, for the part to expand along the die cavity surface. Moreover, this adverse situation can cause a split or crack 36 as seen in FIG. 4.
The present invention solves such problems by prefilling the tubular part to be formed with hydroforming fluid at a relatively low pressure while still outside the die cavity. Moreover, this prefilling is accomplished with hydraulic piston operated seal units which are operable to establish a relatively low sealing capacity relationship with the end of the part for this prefilling and which remain with the prefilled part while the latter is enclosed in a die cavity. The seal units are further operable to then establish a high pressure sealing relationship with the part and provide for hydroforming fluid to then be supplied to the interior of the part through the seal units while the pressure of the fluid is gradually raised to form the part to the die cavity surface. And also while the part is compressed between its ends by the seal units to minimize or prevent thinning of the part. Thus there is eliminated any need for preforming a part to prevent pinching, buckling and splitting or cracking the part. And thus there is no need for preforming equipment and the personnel to operate such resulting in considerable cost, space and time savings.
Furthermore, a simple internal seal is used in the seal units for the prefilling which does not require any significant axial force to be applied to the ends of the part as this seal must only withstand the prefill pressure which, as has been found, may only need to be in the range of 800-1200 psi. On the other hand, an interference fit type seal is effected by the seal units to withstand the high hydroforming pressures which can be 25,000 psi and higher.
It is therefore an object of the present invention to provide a new and improved method and apparatus for preparing and hydroforming parts.
Another object is to provide method and apparatus wherein seal units are used to prefill a part to be hydroformed with hydroforming fluid at low pressure and then use these seal units while the part is in a die cavity for the supply to and pressurizing of the fluid in the part to form the part and also to compress the part between the seal units to minimize or prevent thinning of the part.
These and other objects, advantages and features of the present invention will become more apparent to those skilled in this art by the accompanying drawings and the following detailed description.